IEEE Std 303-2023
IEEE Recommended Practice for Auxiliary Devices for Rotating Electrical Machines in Class I, Division 2 and Zone 2 Locations and Class II, Division 2 and Zone 22 Locations
- Standard No.
- IEEE Std 303-2023
- Release Date
- 2024
- Published By
- Institute of Electrical and Electronics Engineers (IEEE)
- Latest
- IEEE Std 303-2023
- Replace
- IEEE Std 303-2004
- Introduction
In-Depth Interpretation of IEEE 303-2023 Standard
Standard Overview and Technology Evolution
IEEE Std 303-2023 is a key guidance document for the use of rotating electrical auxiliary equipment in hazardous areas within the petrochemical industry. Originally published in 1969, the standard underwent revisions in 1984, 1991, and 2004, was confirmed in 2012, and was most recently revised in 2023. This major change expands the scope of application to include combustible dust hazardous areas within Class II Division 2 and Zone 22, demonstrating the standard's comprehensive response to modern industrial safety needs.
Comparison of Hazardous Area Classification Systems
Area Type Gas Classification Dust Classification Protection Requirements Class I Div 2 Groups A,B,C,D Not Applicable Explosion-proof, Increased Safety, Intrinsic Safety, etc. Zone 2 Groups IIC,IIB,IIA Not Applicable Ex n, Intrinsically Safe, Positive Pressure, etc. Class II Div 2 Not Applicable Groups E,F,G Dust-proof Enclosure, Anti-Ignition Zone 22 Not Applicable Groups IIIC,IIIB,IIIA IP Protection, Surface Temperature Control
Classification of Auxiliary Equipment and Protection Technology
The standard divides auxiliary equipment into two categories: arc/spark generating components and non-arc/sparking components. This classification is based on the potential ignition risk of the equipment and directly affects the selection of protection requirements.
Examples of Arcing/Sparking Components
- Commutators or slip rings
- Bearing temperature switches or relays
- Winding temperature switches
- Vibration switches
- Sliding contact shaft grounding devices
Examples of Non-Arcing/Sparking Components
- Resistance Temperature Detectors (RTDs)
- Qualified space heaters
- Solid-state speed sensors
- Surge capacitors
- Current/voltage transformers
Critical Equipment Installation Requirements
Junction Box Design Specifications
For auxiliary junction boxes that do not contain arcing equipment, the standard does not require explosion-proof or flameproof construction. The design of the junction box is primarily determined by the environmental conditions rather than the hazardous area classification. It is important to ensure that the junction box provides adequate mounting and wiring space and is easily accessible for inspection and maintenance.
Special Requirements for Space Heaters
In Class I Division 2 areas, the surface temperature of space heaters must not exceed 80% of the auto-ignition temperature (AIT) based on an ambient temperature of 40°C. For example, for an AIT of 200°C, the maximum surface temperature is 160°C. The heater nameplate must clearly indicate the power rating, voltage, number of phases, current, and the maximum surface temperature at rated voltage.
Surge Protection Device Selection
Gapless metal oxide arresters (MOVs) and sealed surge capacitors are recommended. These devices can be installed in general-purpose enclosures, but other types of surge protection devices require explosion-proof enclosures meeting Class I Division 1 or Zone 1 requirements.
Wiring Methods and Material Requirements
The standard clearly distinguishes between factory-installed internal and external wiring requirements. Internal wiring is not subject to NEC wiring regulations, but users are advised to inform the manufacturer of specific environmental considerations. External wiring must fully comply with the wiring methods and installation requirements for the corresponding hazardous area.
Implementation Recommendations and Engineering Practices
Equipment Selection Considerations
When selecting auxiliary equipment, engineers should consider the following factors: area classification level, equipment protection type, environmental conditions, maintenance requirements, and cost-effectiveness. Pay particular attention to the consistency of equipment certification markings: NEC Article 505 equipment is marked "AEx," while CE Code Section 18 equipment is marked "Ex."
Installation Practice Points
- Multi-stage equipment requires additional equipotential bonding to prevent arcing between stages
- Space heaters should be de-energized when the equipment is in operation
- Bearing oil heaters should prevent oil carbonization or the generation of hazardous vapors
- Shaft grounding devices should be protected by explosion-proof or pressurized enclosures to protect sparking components
Maintenance and Inspection Requirements
Regular inspections should include verification of equipment integrity, temperature monitoring, maintenance of the protection rating, and legibility of certification markings. When replacing components, ensure that the temperature characteristics of the new equipment do not exceed the specified values of the original equipment.
Real-World Example of a Standard Application
In a refinery pump station application, motor auxiliary equipment in Zone 2 utilizes Ex n-certified space heaters and intrinsically safe temperature sensors. The junction box has an IP54 protection rating, and the internal wiring utilizes oil- and chemical-resistant cable materials. Surge protection uses sealed MOV lightning arresters, which are installed directly in the motor terminal box.
In the Zone 22 area of the grain processing plant, the surface temperature of the space heater is controlled below 165°C to prevent dust carbonization. All equipment enclosures are dust-proof to prevent dust from entering and posing an ignition risk.
Compliance and Certification Requirements
Equipment compliance needs to meet the multiple requirements of NEC, CE Code and IEC standards. Manufacturers should provide complete certification documents, including explosion-proof certification, temperature code marking and installation instructions. It is the user's responsibility to ensure that the installation complies with all applicable regulatory requirements.
The standard emphasizes that IEEE standards themselves do not guarantee safety, and implementers and users are responsible for determining and complying with all appropriate safety practices and applicable laws and regulations. This reflects the standard's position as an engineering guidance document rather than a mandatory regulation.
IEEE Std 303-2023 Referenced Document
- CSA C22.1 2021 Canadian Electrical Code
- IEC 60079-0 Interpretation Sheet 2 - Explosive atmospheres - Part 0: Equipment - General requirements
- IEC 60079-1 Interpretation sheet 1 - Explosive atmospheres - Part 1: Equipment protection by flameproof enclosures "d"
- IEC 60079-10-1 Corrigendum 1 - Explosive atmospheres - Part 10-1: Classification of areas - Explosive gas atmospheres
- IEEE Std 1349 IEEE Guide for the Application of Electric Machines in Zone 2 and Class I, Division 2 Hazardous (Classified) Locations - Corrigendum 1
- NFPA 496 Standard for Purged and Pressurized Enclosures for Electrical Equipment
- NFPA 70 National Electrical Code® Errata 70-23-4, Reference: 701.12(I)
- UL 121201 UL Standard for Safety Nonincendive Electrical Equipment for Use in Class I and II Division 2 and Class III Divisions 1 and 2 Hazardous (Classified) Locations*， 2025-09-25 Update
- UL 60079-0 UL Standard for Safety Explosive atmospheres �C Part 0: Equipment �C General requirements*， 2025-09-25 Update
- UL 60079-1 Standard for Explosive Atmospheres – Part 1: Equipment Protection by Flameproof Enclosures “d”*， 2024-04-22 Update
- UL 60079-2 UL Standard for Safety Explosive Atmospheres - Part 2: Equipment Protection by Pressurized Enclosure "p"*， 2025-09-25 Update
- UL 913 UL Standard for Safety Intrinsically Safe Apparatus and Associated Apparatus for Use in Class I@ II@ and III@ Division 1@ Hazardous (Classified) Locations (Eighth Edition; Reprint with Revisions Through and Including October 16@ 2015)
IEEE Std 303-2023 history
- 2024 IEEE Std 303-2023 IEEE Recommended Practice for Auxiliary Devices for Rotating Electrical Machines in Class I, Division 2 and Zone 2 Locations and Class II, Division 2 and Zone 22 Locations
- 2004 IEEE Std 303-2004 IEEE Recommended Practice for Auxiliary Devices for Rotating Electrical Machines in Class I, Division 2 and Zone 2 Locations
- 1991 IEEE Std 303-1991 IEEE Recommended Practice for Auxiliary Devices for Motors in Class I, Groups A,B,C, and D, Division 2 Locations
